1. Field of the Invention
The present invention relates to a substrate for use in a surface acoustic wave device, and more particularly to a substrate comprising a sapphire single crystal substrate consisting of .alpha.-Al.sub.2 O.sub.3 and an aluminum nitride single crystal layer formed on a surface of the sapphire single crystal substrate by a metal organic chemical vapor deposition (MOCVD).
The present invention also relates to a method of manufacturing the above mentioned substrate for use in a surface acoustic wave device as well as a surface acoustic wave device comprising the above mentioned substrate.
2. Related Art Statement
Heretofore, substrates of surface acoustic wave devices have been generally made of quartz, LiNbO.sub.3, LitaO.sub.3, Li.sub.2 B.sub.4 O.sub.7 and others. These substrate materials have been utilized owing to a reason that they have excellent electromechanical coupling coefficient K.sup.2 and temperature coefficient of delay time (TCD) which are important transmission properties of the surface acoustic wave device. On the other hand, the application of the surface acoustic wave devices has become wider and wider, and it has been required to provide a surface acoustic wave device having a very high operation frequency. However, a propagating velocity of the surface acoustic wave in the above mentioned substrate materials is about 3000-5000 m/sec, and in order to realize a surface acoustic wave device having an operation frequency in the order of GHz, it is required to provide a substrate having a propagating velocity for the surface acoustic wave not lower than 5000-6000 m/sec.
As stated above, in order to realize a surface acoustic wave device having a very high operation frequency, it is necessary to use a substrate having a high propagating velocity. For this purpose, it has been proposed to use a substrate made of aluminum nitride (AlN). An electromechanical coupling coefficient K.sup.2 of the aluminum nitride substrate is about 0.8% which is higher than that of quartz by about five times, and a temperature coefficient of delay time TCD of the aluminum nitride substrate is not larger than 20 ppm/.degree.C. However, aluminum nitride has a very high melting point, and therefore it is difficult to obtain a large bulk aluminum nitride single crystal. Due to this fact, in general, an aluminum nitride single crystal layer is formed on a sapphire single crystal substrate made of .alpha.-Al.sub.2 O.sub.3. Such a sapphire single crystal substrate has been used owing to a reason that it is easily available and its lattice constant does not differ largely from that of the aluminum nitride.
As stated above, it has been proposed to use the substrate in which the aluminum nitride layer is deposited on the sapphire substrate. The inventors of the instant application have conducted various experiments, in which after performing an initial nitriding treatment by exposing an R(1-102) surface of a sapphire substrate to an atmosphere of ammonia to form a very thin aluminum nitride single crystal film, an aluminum nitride single crystal layer is deposited on the aluminum nitride single crystal film by the metal organic chemical vapor deposition (MOCVD). For instance, a sapphire single crystal substrate was placed in a CVD apparatus, and then trimethylaluminum (TMA) and ammonia (NH3) were introduced into the CVD apparatus to deposit an aluminum nitride single crystal layer on the sapphire substrate. It was confirmed that the aluminum nitride single crystal layer thus formed by the MOCVD method has a good electromechanical coupling coefficient K.sup.2.
In the experiments, use was made of a very small sapphire substrate of a square shape having a side of 5 mm. In order to manufacture substrates for surface acoustic wave devices on a practically acceptable large scale, it is necessary to use a sapphire wafer not less than two inches (50.8 mm). That is to say, an aluminum nitride single crystal layer has to be formed on a surface of a two inch sapphire single crystal wafer, then a desired electrode pattern has to be formed on the aluminum nitride layer, and finally the sapphire single crystal wafer has to be divided into chips by slicing. This process is similar to that of manufacturing semiconductor devices on a mass production scale.
In one of the experiments conducted by the inventors, use was made of a two inch sapphire single crystal wafer having a thickness of 300-500 .mu.m, a first aluminum nitride single crystal layer was formed on the sapphire wafer by means of the above mentioned initial nitriding treatment, a second aluminum nitride single crystal layer was formed on the first aluminum nitride single crystal layer having a thickness not less than 1 .mu.m by means of the above mentioned MOCVD process, and finally the sapphire single crystal wafer was divided into a number of surface acoustic wave devices. In the final device, the above first and second aluminum nitride single crystal layers were united together to form a single aluminum nitride single crystal layer. It has been experimentally confirmed that a number of clacks were formed in the aluminum nitride single crystal layer with a mutual spacing of about 1 mm. Surface acoustic wave devices were manufactured using the thus obtained substrates. Then, it was experimentally confirmed that propagation loss of the thus obtained surface acoustic wave devices was very large and the property of the device is deteriorated. In this manner, it is experimentally confirmed that practically usable surface acoustic wave devices could not be manufactured by using the above mentioned sapphire single crystal substrate.
In a field of manufacturing light emitting semiconductor devices, it has been known to use a substrate including a sapphire single crystal substrate and a III-V compound single crystal layer such as GaN and AlN single crystal layer formed on the sapphire single crystal substrate. In order to prevent clacks from being formed in the III-V compound single crystal layer, it has been proposed to form a thin buffer layer on the sapphire single crystal substrate prior to the formation of the III-V compound single crystal layer. The inventors have introduced this method in the formation of substrate for the surface acoustic wave devices. That is to say, a very thin buffer layer consisting of an aluminum nitride single crystal film having a thickness of about 5-50 nm was first formed on a sapphire single crystal substrate, and then a thick aluminum nitride layer was formed on the buffer layer. In this case, during the formation of the relatively thin first aluminum nitride single crystal layer, a surface temperature of the sapphire substrate was kept to a lower temperature such as 300-450.degree. C., and then the substrate was heated to 900-1100.degree. C. during the formation of the relatively thick second aluminum nitride single crystal layer. The thus formed aluminum nitride layer was free from clacks. However, its electromechanical coupling coefficient K.sup.2 has become to substantially zero and the aluminum nitride single crystal layer looses the piezoelectric property. It is apparent that such a substrate could never be used as the substrate for the surface acoustic wave device. In the light emitting semiconductor device, the lost of the piezo-electric property does not matter at all, but in the surface acoustic wave device, the piezo-electric property is indispensable. A reason for disappearing the piezo-electric property by providing the buffer layer could not yet be clarified, but upon observing the microstructure of the surface of aluminum nitride single crystal layer, it has been confirmed that many twins were formed in the surface.